The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of achieving simultaneous fat suppression and T1 inversion recovery contrast in magnetic resonance (MR) imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Inversion recovery (IR) magnetization preparation is an imaging technique that is often used to capture T1 contrast in MR images. Inversion recovery magnetization preparation typically includes the application of an initial 180 degree RF pulse to invert the longitudinal magnetization of tissue. Thus, the T1 relaxation times of the tissue are emphasized by having the longitudinal magnetization recover from a maximal −Mo to +Mo (where Mo is the equilibrium magnetization). A potential drawback of IR magnetization preparation is that data acquired from fat can cause a relatively bright signal to appear in the image due to the shorter T1 time of fat relative to other tissues. This can complicate identification of lesions and other pathologies. This can be particularly problematic for myocardial delayed enhancement (MDE) imaging as well as abdominal imaging where contrast-enhanced lesions are iso-intense with fat.
Myocardial delayed enhancement imaging is a technique that is used to assess the viability of myocardium. The presence of hyper-enhancement following a bolus of gadolinium contrast media indicates the presence of myocardial infarction. One method for MDE utilizes a non-selective (NS) IR preparation segment with a segmented ECG-gated gradient echo (GRE) readout. In these images, fat is usually of high signal intensity. This leads to increased motion-related artifacts from ghosting of fat from the chest wall if the patient is unable to maintain a breath-hold or possibly more significant aliasing artifacts from fat within the shoulder when fields-of-view (FOVs) are small. Furthermore, bright signal from pericardial fat may obscure proper identification of enhancing infarcted tissue over suppressed myocardium, which may be particularly problematic in regions of thinned myocardium such as in areas of chronic infarction or the right ventricle.
As such, for MDE and other scans that include IR magnetization preparations, a fat suppression pulse is used to improve lesion conspicuity by reducing or suppressing the bright fat signal. Typically, the spectrally selective, fat suppression pulse is played out immediately prior to data acquisition. As such, the desired T1 contrast is obtained by a choice of the effective IR interval time, TIeff. The effective IR interval time is generally determined from both the physical delay of the start of the image acquisition segment from the IR pulse and the time from the start of the imaging segment to the acquisition of the central k-space view. It is well-known that the flip angle of the fat suppression RF pulse and the time from that pulse to the acquisition of the central k-space view can be adjusted for optimal fat suppression. It is preferred that the longitudinal magnetization of fat be at or near zero during the acquisition of the central k-space views for optimal fat suppression.
A conventional SPEC-IR (spectrally selective IR) pulse sequence often used in MDE studies is illustrated in FIG. 1. As shown, the pulse sequence 2 is segmented into an IR magnetization preparation segment 3 and an acquisition segment 4. The preparation segment 3 is defined by an IR preparation RF pulse 5 and a spectrally selective (fat) suppression RF pulse 6. Ideally, the fat longitudinal magnetization is nulled at the acquisition of the central k-space views 7 which is coincident with the effective TI time for optimal fat suppression. The time period observed between application of the fat suppression RF pulse and the acquisition of the central k-space views is referred to as TIfat.
A disadvantage of this imaging technique for fat suppression is that it is dependent upon the longitudinal magnetization of fat being nulled at TIfat from the fat suppression RF pulse. Moreover, this nulling is dependent on the longitudinal fat magnetization at the time the fat suppression RF pulse 6 is applied as the IR pulse 5 is non-selective. This is mathematically illustrated in Eqns. 1-4 below. Given t=0 is the time the IR pulse 5 is applied and at t=T1eff is the time that central k-space views are acquired, then the fat signal that is obtained is a function of the longitudinal magnetization of fat at t=T1eff. The effective inversion time is defined as the time from the peak of the IR pulse to the acquisition of the central k-space views in the image acquisition segment.
The fat signal intensity can be thus written as:SIfat∝−Mz,fat exp(−TIfat/T1fat)+Mo(1−exp(−TIfat/T1fat))  Eqn. 1,where T1fat is the T1 value for fat and Mz,fat is the longitudinal magnetization of fat at t=(TIeff−TIfat), and is given by:Mz,fat=Mzeq,fat exp(−tfatsat/T1fat)+Mo(1−exp(−tfatsat/T1fat  Eqn. 2.Mzeq,fat denotes the equilibrium longitudinal magnetization of fat at the time the IR pulse is applied and accounts for the recovery of magnetization after the end of the image acquisition segment, and tfatsat denotes the time when the fat suppression pulse is played out. For optimum nulling of fat signal,Mz,fat exp(−TIfat/T1fat)=Mo(1−exp(−TIfat/T1fat))  Eqn. 3, andTIfat=T1,fat ln((Mz,fat+Mo)/Mo)  Eqn. 4.For fat to be effectively suppressed, TIfat=TIeff−tfatsat. As the time is dependent on Mz,fat, it can be difficult to determine a solution for TIfat or that the solution changes from patient to patient, depending on the variation in Mz,fat according to Eqn. 2. As such, this conventional approach is not reliable for fat suppression in an IR T1-weighted acquisition.
However, as T1 contrast imaging is a preferred technique to identify lesions in abdominal and myocardium imaging, it is desirable to not perturb the true inversion time, TI, and the effective inversion time, TIeff. This is because, depending on the TI used and other sequence parameters, there may not be an optimal TIfatsat to null fat without having to change the primary TI time.
One proposed technique for providing fat suppression while maintaining T1 contrast is to use a spectrally selective inversion pulse set to the water peak and a spectrally selective inversion pulse set to fat. A drawback of this double spectrally selective IR scheme is that if there is substantial magnetic field inhomogeneity, blood outside the imaged slice may be not sufficiently inverted. This can contribute to image artifacts and, in particular, for a myocardial viability study, bright signal in the ventricle.
It would therefore be desirable to have a system and method capable of IR imaging with fat suppression that maintains the benefits of IR magnetization preparation. It would also be desirable to have an imaging technique that globally or uniformly inverts blood magnetization.